Ancient Islamic tradition speaks of the Djinn, powerful deity-like entities that are born of smokeless fires and dwell in the Kaf, a mythical mountain range said to circle the Earth. There are many types of Djinn, and sub-types that can be defined by their colour. Most importantly, are the Jann, or perhaps the Ghul.
Recorded history of the Djinn began some thousands of years ago, and with all things written by 'ancient' man, the accounts were cryptic, embellished, and fraught with mysticism and fable. The Jann are described as a mostly benevolent type of Djinn, shape-shifters who lived in the desert, and take the forms of whirlwinds and white camels. They were believed to be open-minded about humans, and were among the first Djinn encountered by people, and were sometimes attributed to leading those travelers they liked to oases. They often appeared as tall, dark robed figures that floated gracefully along the sand, leaving no tracks and untouched by the desert winds.
The Ghul were much the opposite of the Jann; shapeshifters also, who could disguise themselves as mortal men and women to better lure their prey away from caravans or villages. The Ghul were nocturnal creatures who inhabited graveyards, ruins and other lonely places. Sometimes they are described as dead humans who sleep for long periods in secret graves, then awake, rise and feast on both the living and the dead. Ghul also personify the unknown terrors held by the desert.
The stories of old describe the Jann and the Ghul as mortal enemies, but some historians have had other theories. That the Ghul and the Jann were one and the same, and the differences were a simple matter of one's view on the subject. The allied Djinn were referred to as Jann, while those that were controlled by the enemy were the Ghul's.
The Jann appear to be semi-incorporeal beings, or perhaps they are only partly phased with our world. Conventional weapons can be effective against them, but it's really a 'hit and miss' application; some rounds will find something solid to strike, others seem to pass straight through. They are weakest in bright light, and strongest in the dark.
For an example with an encounter with the Jann / Ghul, see Hood's biography.